Flow meters and flow meter systems are utilized to measure properties of fluids flowing in pipes or conduits. One particular property of interest is the flow rate or velocity of the fluid. Ultrasonic transducers are often employed to measure flow velocity based on the transit time of ultrasonic signals transmitted through the fluid.
Typically, a flow meter measures flow velocity using two transducers which are located upstream and downstream from each other on a pipe or conduit. Each transducer transmits and receives in an alternating manner. Flow velocity is then computed as a function of the transit times of the ultrasonic signals both with and against the direction of flow, i.e. the difference between the upstream and downstream transit times. For example, in the presence of a flow, the transit time signal pulse transmitted from the upstream transducer to the downstream transducer is faster than the pulse transmitted from the downstream transducer to the upstream transducer. By subtracting the transit time from the downstream transducer to the upstream transducer from the transit time from the upstream transducer to the downstream transducer, the transit time differential Δt is calculated. Utilizing other known parameters such as the conduit dimensions and ultrasonic signal path length, and the measured speed of sound in the fluid, flow velocity can thus be determined. The general principle of transit time measurement in ultrasonic flow meters is fairly well known.
Fluid flow measurements often provide the basis for cash transfers between businesses, where sales of resources may ultimately depend on the accuracy of flow rate measurements. Moreover, accurate flow rate measurement can provide valuable information about industrial processes such as chemical or combustion processes, and the information can serve as the basis for optimization and control. For instance, once fluid flow velocity is calculated other fluid qualities may also be determined, such as the volumetric flow rate Q, which is a function of velocity and pipe area, and/or the mean molecular weight of the fluid. The latter quality is a good indicator of the efficiency of certain chemical processes. By monitoring this quality, chemical processes can be optimized. In one example, the assignee herein provides an ultrasonic flow meter, Model No. GF868, to monitor flare gas emitted into the atmosphere. This flow meter is generally used to measure flow rate for chemical processing industries. With better than 5% accuracy, it provides an efficient, cost-effective and accurate means to monitor flare gas emissions.
Certain states such as Texas and California, however, as well as certain European countries, require that flare gas flow meters be calibrated at least annually to assure that emissions are well monitored and controlled. There are other circumstances where calibration of flow meters in general may also be required or desired. Additionally, in many typical flow meters, erosion or contaminant deposits over time can cause the flow meter transducer signals to deteriorate, which can result in less precise flow rate measurements.
It is typically cost prohibitive to remove a spool piece or portion of a pipe for offsite calibration, because of the great expenditure of time and resources and resulting system down time.
One known approach to calibrating flow meters includes use of calipers and a CMM (Coordinate Measuring Machine) to correct speed of sound measurements. Zero calibration of the flow meter is effected by removing the flow meter transducers from the conduit flow line and placing them into a “black box” with no flow. This method is cumbersome, however, and difficult to use in the field.
Also, as a practical matter it may not be possible to calibrate the flow meter across the limits of a system's specified flow velocity range, for example at the maximum flow rate—which can be as high as 300 feet/second—because such conditions may not be present during normal operation or at the time of calibration.